Aluminum is produced by the electrolytic reduction of alumina from an electrolyte. The electrolyte comprises primarily molten cryolite containing alumina and possibly other materials such as fluorspar, dissolved therein. The classic prebaked anode and Soderberg anode aluminum reduction cells comprise an anode structure suspended in a cryolite bath. Beneath the cryolite bath is a molten aluminum metal pad which forms the cathode and collects on the carbon blocks in the bottom of the cell.
The cathode blocks conduct the electric current from the molten metal pad to the external electric bus system of the cell. In a typical cathode design, multiple steel collector bars extend from the external bus bars through each side of the cathode shell into the carbon cathode blocks. The cathode collector bars are usually tightly attached to the cathode blocks with cast iron or carbon cement to enhance the electrical contact between the carbon cathode blocks and the cathode collector bars.
Modern aluminum manufacturing plants often have hundreds of electrolytic cells with over one-hundred cells connected in series. With such a large number of cells, cell maintenance is an ongoing operation involving numerous personnel and heavy equipment, such as cranes, to move the heavy carbon cathode blocks and cathode collector bars. Aluminum reduction cells of this type are operated at low voltages (e.g. 4-4.5 volts) and high currents (e.g. 70,000-275,000 amps). The current enters the reduction cell through the anode structure and then passes through the cryolite bath, down through the molten aluminum pad where it then enters the carbon cathode blocks and is carried out of the cell by the cathode collector bars.
As the electrolyte bath is traversed by electric current, alumina is reduced electrolytically to aluminum at the cathode, and carbon is oxidized to carbon dioxide at the anode. The aluminum thus produced, accumulates in a molten aluminum pad and, in a conventional cell, is tapped off periodically.
The alumina-cryolite bath is typically maintained on top of the metal pad at a set depth. There is a voltage loss as the current passes through the cryolite bath. This voltage loss is directly proportional to the length of the current path and is typically about 1 volt per inch of gap between the anode and molten aluminum pad, i.e. the interpolar distance. Therefore, any restriction on the reduction of the anode to cathode spacing restricts the achievement of maximum power efficiency and limits the ability to improve the electrolytic cell operation. The molten aluminum pad acts as a liquid metal cathode in common commercial cells.
Commercial aluminum reduction cells are generally operated by maintaining a minimum depth of liquid aluminum on the floor of the cell which is "seen" by the bath as the true cathode. This minimum aluminum depth is usually at least 2 inches and may be 20 inches.
The high currents passing through the electrolytic cell produce powerful magnetic fields that induce excessive circulation in the molten aluminum pad leading to problems such as reduced electrical efficiency and "back reaction" of the molten aluminum with the electrolyte.
The flow of electrical current through the aluminum pad and carbon cathode naturally follows the path of least resistance. The electrical resistance in a conventional cathode collector bar is proportional to the length of the current path from the point at which electric current enters the cathode collector bar to the nearest external bus. As generally depicted in FIG. 1, the lower resistance of the current path starting at points on the cathode collector bar closer to the external bus causes the flow of current through the molten aluminum pad and carbon cathode blocks to be skewed in that direction. The horizontal components of the flow of electric current interact with the vertical component of the magnetic field, adversely affecting efficient cell operation. These interactions increase the motion of the metal pad, sometimes violently stirring the molten pad and generating vortices, and causing localized electrical shorting to the anode. The magnetic fields also lead to unequal depths in the molten aluminum pad and the cryolite bath.
Metal pad turbulence can also increase the "back reaction", or reoxidation, of cathodic products, thereby lowering cell efficiency. Furthermore, metal pad turbulence tends to accelerate distortion and degradation of the cathode bottom liner through attrition and penetration of the cryolite.
The depth variations also restrict the reduction of the anode to cathode gap and produce a loss in current efficiency since power is lost to the electrolyte interposed between the anode and cathode blocks. Movement of the molten aluminum metal pad also tends to cause uneven wear on the carbon cathode blocks and may result in early cell failure.
It is possible to reduce molten aluminum metal pad stirring by modifying the bus system on an existing cell line to reduce the overall magnetic effects. However, it is normally very expensive to modify the bus system.
In recognition of the adverse effects that horizontal current components have on cell efficiency, cell designs have been proposed which attempt to reduce the horizontal component of current by changing the basic design of the cathode collector bars. The proposals found in the literature, however, often fail to account for the practical necessity of preassembling cathode blocks onto the iron collector bars so that the carbon cathode blocks can be reassembled in the bottom of the cell. They also fail to provide designs which are amenable to safe handling by maintenance crews using heavy equipment such as cranes.
One example of a design of an aluminum reduction cell which attempts to increase cell efficiency by reducing horizontal current components is found in U.S. Pat. Nos. 4,194,959 to Hudson, et al. and 4,592,820 to McGreer. The Hudson et al. patent teaches the use of one or more collector bars. Each collector bar is provided with one or more connector bars. The connector bars carry the current from the collector bars to the external bus system. The connector bars are of a lighter gauge material than the collector bars and are connected to the collector bars at points distant from the ends of the collector bars. The resistances of the disclosed connector bars are chosen so that preselected currents are drawn from each corresponding collector bar section. This design fails to account for the practical necessity of preassembling cathode blocks onto the iron collector bars so that the cathode shells can be relined. In addition, this design calls for major changes in the design of conventional cathode bars mandating a new cathode shell and current bus requiring major capital investments. Such a design would also be inherently weak due to the lighter gauge material used in the connector bars and probably could not safely be handled by workers and cranes during maintenance operations.
U.S. Pat. No. 2,528,905 of Ollivier, et al. teaches disposing "current lead bars" perpendicular to the bottom of the electrolytic cell. This design, which necessitates providing passages through other portions of cell lining, i.e. the concrete vault and the layer of insulating bricks, would require extensive capital expenditures since it entails a significant departure from the conventional rectangular-block collector bar design.
U.S. Pat. No. 3,787,311 to Wittner et al. attempts to equalize the current flow through the cell bottom by providing a plurality of carbon blocks with different resistivities. The carbon blocks are arranged such that blocks with higher resistivities are closer to the sides of the cell where the current flow would otherwise be greater. It is generally known to those skilled in the art that high resistivity blocks will convert to lower resistivity blocks during the high operating temperatures. Accordingly, the use of different resistivity blocks does not provide the desired result.
U.S. Pat. No. 2,868,710 to Pontremoli discloses a design modification for the cathodic bottom of electrolysis furnaces which results in a portion of the (cathode) block being situated under the cathode conductor (collector bar). This design would provide a cell that is very difficult to construct and maintain in comparison to typical cells wherein the carbon cathode blocks are provided with grooves that allow the relatively simple lowering of the carbon cathode block onto the cathode collector bar for subsequent sealing.
Prior attempts to solve the recognized current distribution problem in aluminum electrolytic reduction cells fail to proiide a practical design which can be implemented without major capital expenditures and which is safe to handle by maintenance operators using heavy equipment.
The present invention provides a more practical approach by using specifically-designed cathode collector bars to minimize the horizontal electrical currents in the metal pad. The improved cathode collector bar of the present invention is advantageously employed in existing cell designs using standard carbon cathode blocks or carbon cathode blocks having new improved molten alumina wettable surfaces such as described in U.S. Pat. No. 4,526,911 to Boxall et al. The improved collector bar design of the present invention can also be advantageously employed in new low energy, drained sloped cathode cells such as described in U.S. Pat. No. 4,602,990 to Boxall et al.